Abstract: The present invention provides a cataplasm base that exhibits high water content, having adhesive strength and shape-retaining property, that is free from paste seepage, and that prevents skin irritation and can be cleanly detached from the skin without paste residue; and a cataplasm using the same. The inventive cataplasm base includes as indispensable components a water-soluble copolymer in the form of fine powder, the water-soluble copolymer including 1 to 30 mole percent of 2-acrylamido-2-methylpropane sulfonic acid and 70 to 99 mole percent of acrylic acid as monomer units, wherein 20 to 60 mole percent of the acids is neutralized, and wherein the content of unreacted monomers is 0.5 weight percent or less; a polyhydric alcohol; a polyvalent metal compound; and water. A dermatological cataplasm including the cataplasm base is also provided.